I agree with Naomi,

the convert is unneccesary and might be causing performance issues.
Also, it the date ranges used in practice vary greatly, you might want to add "WITH RECOMPILE" to either the call or the stored procedure.
https://docs.microsoft.com/en-us/sql/relational-databases/stored-procedures/recompile-a-stored-procedure?view=sql-server-ver15

This will ensure the execution plan is determined on the actually current values of the parameters at each individual call.

Walter,




>

>DECLARE @startdate DATETIME
>DECLARE @enddate DATETIME
>select @startdate = '2020-03-31'
>select @enddate = '2020-03-31'
>
>select inv_number, billingname, bill_store, sum(charged) as charged, sum(storetax) as tax, sum(charged + storetax) as totalcharge from (  select  IIF(right(b.bill_store,6) = SPACE(6), c.name, s.store_name) AS billingname, b.* from
>  (SELECT inv_number, dbo.gettaxrate(tax_area) AS taxrate, dbo.gettaxareaname(tax_area) AS taxareaname, bill_type, bill_store, tax_area, trancode, charged, storetax, storetxbl  FROM billing 
> where CONVERT(date,inv_date) >= @startdate AND CONVERT(date,inv_date) <= @enddate
> UNION ALL SELECT invno AS inv_number, dbo.GetTaxRate(1) as taxrate, dbo.gettaxareaname(1) AS taxareaname, SPACE(23) as bill_type, custno AS bill_store, 1, 'MI', invamt - tax AS charged, 
> tax as storetax, invamt - tax AS storetxble  from arinvoice  where left(invno,1) = SPACE(1) and CONVERT(date,invdte) >=(@startdate)  AND CONVERT(date,invdte) <= (@enddate)
> ) B INNER JOIN company C ON LEFT(B.bill_store,4) = c.comp_code LEFT JOIN store s ON LEFT(s.storecode,11) = b.bill_store ) i group by inv_number, billingname, bill_store order by inv_number 
>
>

>
>in an SSMQ query this code takes under a second.
>When I convert to code to a stored procedure and pass parameters, it gives the same results, but runs for 90 seconds.
>
>Any ideas?